The present invention relates to displacement on demand engines, and more particularly to a control system for detecting spark during displacement on demand transitions.
Displacement on Demand (DOD) engines deactivate one or more cylinders when full engine power is not needed. Running on fewer cylinders reduces pumping losses and improves fuel economy. An engine control system transitions from a deactivated mode to an activated mode when full power is required or for stability as the engine nears idle.
Spark knock is caused by auto-ignition of a fuel/air mixture in the cylinders. High pressure waves propagate and cause an audible xe2x80x9cknockingxe2x80x9d sound. Audible spark knock causes customer dissatisfaction and can lead to engine damage. Some engine control systems detect spark knock and vary spark advance to reduce spark knock. A knock sensor monitors a knock frequency in each cylinder during part of the power stroke.
The output of the knock sensors provides an instantaneous noise value (INST). Knock occurs when the instantaneous noise value exceeds a knock threshold (TH). The difference between the instantaneous noise value and the threshold determines a knock intensity, which is used to reduce spark. A mean average deviation (MAD) is calculated based on the difference between the average and the instantaneous noise values. The updated MAD values are used to calculate the knock threshold for the subsequent combustion event for the cylinder.
The knock threshold defines a boundary between acceptable noise (no knock) and unacceptable noise (knock). The filtered instantaneous noise (INST) value is used to vary the gain of a band pass filter (BPF). The gain is used to increase or attenuate knock depending on the value of background noise.
An exemplary method for controlling spark knock is shown in FIG. 1. Spark knock control 10 begins with step 12. In step 14, control determines if the engine is operating. If the engine is operating, control measures an instantaneous noise in step 16. If the engine is not operating, control ends in step 52. In step 16, an instantaneous noise value is measured. In step 18, control determines if knock is present. If knock is present, a current average for knock is updated in step 22. If knock is not present, a current average for no knock is updated in step 20. The average calculations are represented by the following exemplary formulas:
For no knock:
AVEcurrent=AVEprior+[(INSTxe2x88x92AVEprior)(FC)]
For knock:
AVEcurrent=AVEprior+[(INSTxe2x88x92AVEprior)(FC)(KM)]
where FC is a detection filter coefficient and (KM) is a knock multiplier. The (KM) is applied to minimize the effect of a large instantaneous value.
If no knock is detected, control determines if the instantaneous noise value is less than the average noise value in step 24. If the instantaneous noise value is less than the average noise value, a new MAD value is calculated in step 26. An exemplary MAD calculation is represented by the following exemplary formula:
MAD=MADPREV(1xe2x88x92Filt Coeff)+(AVEcurrentxe2x88x92INST)(Filt Coeff)
MAD is calculated using a first order lag filter. A new threshold is determined in step 28. An exemplary threshold is represented by the following formula:
TH=AVEcurrent+(MADcurrent)(MADmult)
where MADmult is a MAD multiplier. The MAD multiplier is a function of engine speed and load. In step 30, control determines if knock is present. If knock is present, a current knock gain average is updated in step 36. If knock is not present, a current no knock gain average is updated in step 32. The gain average calculations are represented by the following exemplary formulae:
For no knock:
GAINAVGcurrent=GAINAVGprior+[(INSTxe2x88x92GAINAVGprior)(FCgain)]
For knock:
GAINAVGcurrent=GAINAVGprior+[(INSTxe2x88x92GAINAVGprior)(FCgain)(KMgain)]
where FCgain is a gain average filter coefficient and (KMgain) is a gain average knock multiplier.
In step 40 control determines if GAINAVGcurrent is greater than a maximum GAINAVG threshold. If GAINAVGcurrent is greater than the maximum GAINAVG threshold, the knock signal gain is decreased in step 48 and control returns in step 50. If GAINAVGcurrent is not greater than a maximum GAINAVG threshold, control determines if GAINAVGcurrent is less than a minimum GAINAVG threshold in step 44. If GAINAVGcurrent is less than a minimum GAINAVG threshold, the knock signal gain is increased in step 46 and control ends in step 50. If GAINAVGcurrent is not less than a minimum GAINAVG threshold, control returns in step 50. The equations set forth with respect to AVEcurrent, MADcurrent, and GAINAVGcurrent are hereinafter collectively referred to as xe2x80x9cthe knock equationsxe2x80x9d. The knock equations are updated for each firing event in each cylinder.
Performing knock detection on a DOD engine presents potential drawbacks. When cylinders are deactivated, the running cylinders operate at a higher load, which increases the combustion noise of the running cylinders. While the deactivated cylinders contribute no spark knock noise, background and mechanical noise is detected from the knock sensors that are associated with the deactivated cylinders. The measured noise reduces the average value of the deactivated cylinders. When the deactivated cylinders are reactivated, the threshold is artificially low based on the reduced average value. Acceptable noise may be incorrectly characterized as spark knock, resulting in false retard.
A method according to the invention minimizes false spark knock detection for a displacement on demand engine having activated and deactivated modes. The engine is operated in the activated mode. Knock detection is performed on all cylinders of the engine during the activated mode. The engine is operated in the deactivated mode. Knock detection is performed on activated cylinders during the deactivated mode. Knock detection is disabled for deactivated cylinders during the deactivated mode.
A method according to the invention minimizes false spark knock detection for a displacement on demand engine having activated and deactivated modes. The engine is operated in the activated mode. A knock threshold is established. Knock detection is performed on all cylinders of the engine during the activated mode using the knock threshold. The engine is operated in the deactivated mode. The knock threshold is increased for the transition period. Knock detection is performed on all cylinders of the engine during the deactivated mode using the increased knock threshold.
A method according to the invention minimizes false spark knock detection for a displacement on demand engine having activated and deactivated modes. A noise value is measured in each cylinder of the engine. A threshold knock value is established based on the measured noise value for each cylinder of the engine. One or more cylinders are deactivated. The noise in the deactivated cylinders is frozen and ignored. The deactivated cylinders are reactivated. The threshold knock value is updated for the deactivated cylinders based on the measured noise values from activated cylinders. Knock is determined for the reactivated cylinders based on the updated threshold.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.